Fasteners such as screws and bolts are well known for use in joining contiguous objects. Such elements have longitudinal shanks of circular cross-section topped at one end by a head. The head may have a round, square or hexagonal cross-section with a lateral dimension greater than that of the shank diameter. The shank is threaded along its length to engage either directly with matching internal threads of a bore, as in the case of a screw, or with matching internal threads of a nut received at the free end of the shank, as in the case of a bolt. Threading may extend the full length of the shank or begin after an unthreaded interval or "lag" at a distance below the head.
In a typical screw fastening procedure, a screw is threaded partially into the objects to be connected to mesh with matching threads of a bore either previously created or created by the rotating action of the screw itself. In a typical nut and bolt fastening procedure, a bolt is placed into one end of aligned predrilled bores of adjacent objects until a free end, threaded portion of its shank protrudes from the opposite end of the bores, with the head acting as a stop to limit further inward movement. An internally threaded nut is then brought over the free end of the shank and rotated toward the head to clamp the objects between the bolt head and the nut.
It is known to taper a circular cross-sectioned screw or bolt outwardly from an upper portion of the shank to the head to provide a conical transition between the shank and the head for the purpose of matching the underside of the head to the contour of a correspondingly tapered larger diameter section or counterbore at the insertion end of a bore. It is also known, especially in connection with screws, to taper a threaded lower portion of the shank down to a point at the free end in order to achieve a self-tapping capability.
Ring elements, commonly called "washers" are frequently used adjacent the underside of the head and/or the head side of the nut. These may be used for spacing or to prevent the head or nut from marring the surface of one or both of the objects being fastened. If a fastener might have a tendency to work itself loose, a locking-type washer or similar element may be used in spring-like manner to impart axial tension between the head and nut. A typical lock washer for use where vibration might otherwise unthread a nut from a fastener shank, is a split ring washer which has one end elevated slightly relative to the other in a single turn, coil spring configuration to impart axial tension when the washer is axially compressed between the head or nut and the fastened objects. The spring constant of such a lock washer remains approximately constant when compressed, and provides a generally linear increase in tension force over the region of interest as the fastener is tightened.